1. Technical Field
The invention relates to a sliding element, in particular a piston ring, which exhibits good overall wear resistance as well as favorable tribological properties in the region of the running surface, and to a method for producing the same.
2. Related Art
When reducing the carbon dioxide emissions of internal combustion engines, fuel consumption plays a key role. This is influenced, inter alia, also by the frictional losses of the sliding elements in the engine, in particular in the region of the pistons. The sliding elements, for example piston rings, have running surfaces at which they are in sliding contact with a friction partner. This tribological system is complex and is significantly determined by the material pairing of the friction partners.
Therefore, there is a need for sliding elements in internal combustion engines, which exhibit the most favorable friction behaviour possible throughout the entire service life. However, especially with modern engines, high thermal and mechanical loads occur at the sliding elements.
The sliding elements, e.g. piston rings, pistons or cylinder liners in internal combustion engines are therefore supposed to have low coefficients of friction as well as high resistance to wear throughout a long service life.
Piston rings are known from the prior art, the flanks of which are nitrided in part or in full and the running surfaces of which have a different coating at least in part.
DE 102 21 800 A1 discloses a steel piston ring having a running surface, an inner surface as well as upper and lower flanks provided therebetween, with the running surface being provided at least in part with a thermal spray layer as running surface coating and a nitrided layer created by plasma nitriding being provided at least on the flanks.
U.S. Pat. No. 6,508,473 B1 describes a piston ring having a nitrided layer on the upper and lower flanks or on the upper and lower flanks and the inner circumferential surface, and a hard film formed by ion plating on the outer circumferential surface.
DE 10 2005 023 627 A1 reveals a steel piston ring having a running surface chambered on one side, with the running surface being coated with a chromium-ceramics-based wear protection layer having micro-cracks and at least the flanks being provided with a wear-reducing nitrided layer.
DE 10 2005 011 438 B3 discloses a method for producing wear protection layers on a piston ring base body consisting of steel or case iron, with the running surface region being first provided at least in part with an at least single-layer thermal spray layer on the basis of nitrogen-affine metallic elements, and then at least the flanks and the running surface with the spray layer applied thereto are subjected to a nitriding process.
Even though such sliding elements have layers with satisfactory wear resistance, they either have no running surfaces with particularly low coefficients of friction or no sufficient adhesion between the substrate and the wear protection layer.
Furthermore, DLC coatings on running surfaces of piston rings are known from the prior art. Diamond-like carbon layers (DLC) constitute a metastable form of amorphous carbon with a significant portion of sp3-hybridised carbon, which exhibit particularly favorable tribological properties. A summary of the possible DLC layer systems can be found in the VDI Guideline 2840 Carbon Layers. 
DE 10 2011 003 254 A1 discloses a sliding element where at least one running surface has, from the inside outwards, a coating with an adhesive layer containing metal and a ta-C-type DLC layer having a thickness of at least 10 μm. The basic material of the sliding element may or may not be nitrided.
Such sliding elements have a low coefficient of friction on the running surface, but are not sufficiently resistant to wear.
Known from DE 10 2011 014 483 B3 is a method for producing a steel piston ring in which a base body is produced that is provided with chambering on the side of the running surface, the running surface is directly provided with a wear-resistant layer and the circumferential and flank regions of the base body, which are not provided with the wear-resistant layer, are nitrided. The wear-resistant layer can consist of, inter alia, a DLC layer or a chromium layer, and a chromium layer can also be applied to the DLC layer.
Owing to the direct application of the DLC layer to the substrate of the running surface, however, the wear resistance of the sliding element is limited by the insufficient adhesion of the DLC layer to the running surface.